Jet injection devices administer intramuscular and subcutaneous medications without the use of needles. Among the many advantages of jet injection are the reduction of pain and apprehension associated with needles, the elimination of needle stick injuries and the reduction of environmental contamination associated with needle disposal. Jet injection devices are useful in a wide range of drug therapies including immunization vaccines, hormones and local anesthetics, as well as the administration of insulin to the diabetic population, where individuals may require a number of daily injections. Thus, their use has become of increasing interest, particularly by persons of limited physical ability such as the elderly, or the very young.
The principles of jet injection and the advantages of drug delivery systems over hypodermic needles have long been known. However, very few jet injection devices have achieved commercial success in the marketplace. This lack of acceptance by the user community can be attributed to a number of factors, chief among which are: the high cost of prior art injection devices, complex filling techniques, and mechanical designs which have potential to inflict serious injury if an injector device were inadvertently fired without a medicament container inplace.
One existing needleless injection device is described in U.S. Pat. No. 4,874,367 to Edwards. It employs a sealed ampule that is prefilled with a selected amount of medication. The prefilled ampule is attached to a separate spring-loaded firing mechanism which, when triggered, propels a ramrod out of the front of the mechanism and against a plunger located in the ampule. The ramrod drives the plunger against the medication, producing a high pressure jet for injection purposes. The plunger expels the medication from a discharge orifice into the patient's subcutaneous tissue.
Although effective in some respects, this prior art apparatus is severely limited in practical application. In order to cock the firing mechanism, the user is required to force the ramrod back into the firing mechanism by pushing the device against a solid surface such as a table top until the ramrod latches behind a trigger mechanism. Thus, the strength of an individual user places limits on the spring force that can be developed in the device. A spring force low enough to be practical for the elderly or the very young to cock is typically inadequate to produce reliable jet injection pressures for most adults. In addition, the firing mechanism, having a spring actuated ramrod, which extends outside of the device body, has the potential to inflict serious injury if inadvertently fired without the ampule in place. Consequently, it is desirable to have a needleless injection system that overcomes these concerns.
Moreover, the fixed dose ampule must be prefilled at the factory and then attached to the injector to be administered when required. In practice however, this procedure is not practical, for the simple reason that drug products cannot be stored in plastic containers for the long periods of time typically experienced by a factory prefilled ampule. The only approved material for long term liquid medication storage is type-I glass, which is used for virtually all drug products. However, due to the dynamics of jet injection in which the ampule is subjected to very high pressures during the injection stroke glass is not a suitable material for the ampule because it is easily shattered. Consequently, it is desirable to have a unit dose ampule which may be attached to a conventional glass container at the time of use and filled with an accurate dose of medication. The ampule must be made of a high strength plastic material.
U.S. Pat. No. 4,913,699, to Parsons, describes a needleless injection device having a firing mechanism that operates to release compressed gas from a storage compartment. The compressed gas acts upon a piston which drives a plunger that ejects a selected dosage of medication through an aperture in the discharge end of the device. However, the medication to be administered must first be drawn into a liquid medication containing chamber provided in the interior of the injector before being dispensed. Thus, although being self-cocking, reloadable, and reusable, the device is somewhat complicated. In addition, no provision is made for filling the medication chamber directly from standard medication containers. In order to fill the medication chamber, a complex liquid transfer system is required.
Injectable medications fall into two different categories; namely, unit dose drugs such as vaccines and analgesics and variable dose drugs such as insulin where the dose size must be adjusted to meet the immediate needs of the individual at the time of administration. When a variable dose is required, as in the case of the administration of insulin, a very accurate amount of medication must be transferred to a variable dose ampule. Insulin doses are typically marketed in 3 ml and 5 ml syringe cartridges, as well as provided in bulk in a standard 10 ml medication vial. These dose categories and differing medication source containers, therefore impose conflicting design requirements on ampules or syringe compartments provided in prior art jet injection systems.
U.S. Pat. No. 4,680,027 to Parsons, et al. describes a hypodermic injection apparatus which includes a disposable syringe which can be inserted into a power supply mechanism. The syringe has a cavity for holding liquid medication and an aperture on one end through which liquid medication can flow into and out of the cavity. The syringe has a hollow, tubular needle removably attached to it so that the syringe can be filled by inserting the needle into a container of liquid medication. Liquid is drawn from the container by pulling on a plunger disposed within the syringe cavity which causes a suction to draw liquid from the container and into the cavity in a conventional manner.
The power supply mechanism has a trigger which, when pulled, unseals a container of compressed gas to provide a motive force to force the syringe plunger toward the aperture so that the liquid medication is ejected through the aperture with sufficient force to penetrate the skin.
The above-described power supply mechanism includes safety interlocks, one of which prevents securing the syringe within the power supply mechanism until the needle is removed. A second interlock prevents movement of the trigger so that the gas container cannot be opened unless the syringe is fully secured in the power supply mechanism. Both interlocks are both relatively complex mechanical systems.
Consequently, there is a need for a needleless hypodermic injection device which has an enhanced simplicity of design, including a simple design of the interlock mechanisms. Furthermore, there is a need for a needleless hypodermic injection system which includes a medication ampule which may be rapidly and conveniently attached to a variety of medicament sources and which extracts accurate doses of medication therefrom without the risk of injury due to needle use or the contamination hazards attendant with needle disposal.
In this regard, it is desirable to have a convenient needleless jet injector device that is dimensioned and arranged to be grasped in the hand of a user. The system should comprise both a permanent injector firing mechanism and a disposable ampule cooperating in a novel design having simplicity in both structure and function. The disposable ampules should be such that the user can fill it with a selected dosage conveniently and accurately from existing medication vials. The injector firing mechanism should be conveniently and safely operated without the need for a user to force the apparatus against a piece of furniture in order to cock a spring.